feat(workspace): route platform Back gestures to the header Back (#1943)

* feat(workspace): route platform Back gestures to the header Back

The mouse Back button, Alt+Left (Cmd+[ on macOS) and, in Electron, the
window's app-command browser-backward (Windows/Linux) and trackpad swipe
(macOS) now run the workspace header's Back, one step each.

- WorkspaceBackGesturesService (shell-provided) dispatches every gesture
  through WorkspaceBackNavigationService.goBack(). A gesture is swallowed
  while a dialog, menu, the phone drawer or a fullscreen player is up, and
  left to the browser only where the header has no Back.
- The chord is a navigate-back entry in the shortcuts dialog, handled in a
  capture-phase listener of WorkspaceKeyboardShortcutsService; ignored while
  typing, repeats swallowed.
- Electron forwards app-command and swipe over the push-only channel
  WINDOW:BACK_REQUESTED (preload onWindowBackRequested), gated in the
  renderer on RuntimeCapabilitiesService.supportsWindowBackGestures.
- A mouse press that also arrives as app-command goes back once, however
  long the button is held; Electron's own Chromium history step on the
  mouse button is always prevented.
- No Forward binding.

Docs: workspace-shell (Platform gestures), portal-detail-navigation,
electron-security, UI guidelines, README shortcuts table.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* refactor(workspace): drop NgZone from the Back gestures service

Master runs zoneless change detection and no longer uses NgZone; the
bridge callback calls goBack() directly.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* fix(workspace): keep Back gestures where the Fullscreen API is missing

iPhone Safari has no unprefixed document.fullscreenElement; the
undefined value read as "a player is fullscreen" and blocked every
gesture.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* fix(workspace): keep a held Back chord's repeats swallowed after Back

Repeats asked for the current result, so once the first press reached a
page with no Back left they were released to the browser, which could take
another history step. Repeats now follow the chord's first press until a
key is released.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* fix(electron): one Back per press of the Linux Browser Back key

On Linux Electron raises app-command browser-backward for every
unhandled event of the Browser Back key (NativeWindowViews::
HandleKeyboardEvent), keydown and keyup alike, so one press went back
twice. The main process now consumes that key in before-input-event and
requests Back once per press; mouse Back buttons still arrive as
app-command.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: 4gray <fourgray@proton.me>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

.changes/workspace-back-gestures.md

@@ -0,0 +1,6 @@
+---
+type: feature
+area: workspace
+---
+
+The mouse Back button, Alt+← (Cmd+[ on macOS) and, in the desktop app, the macOS trackpad swipe now do the same as the Back button in the top bar. Each goes back one page, and the keyboard shortcut is listed in the shortcuts dialog.